linux-kernel - [PATCH 13/28] mm, page_alloc: Remove redundant check for empty zonelist

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Message-Id: <1460711275-1130-1-git-send-email-mgorman@techsingularity.net>
Date:	Fri, 15 Apr 2016 10:07:40 +0100
From:	Mel Gorman <mgorman@...hsingularity.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Vlastimil Babka <vbabka@...e.cz>,
	Jesper Dangaard Brouer <brouer@...hat.com>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...hsingularity.net>
Subject: [PATCH 13/28] mm, page_alloc: Remove redundant check for empty zonelist

A check is made for an empty zonelist early in the page allocator fast path
but it's unnecessary. When get_page_from_freelist() is called, it'll return
NULL immediately. Removing the first check is slower for machines with
memoryless nodes but that is a corner case that can live with the overhead.

Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
---
 mm/page_alloc.c | 11 -----------
 1 file changed, 11 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index df03ccc7f07c..21aaef6ddd7a 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3374,14 +3374,6 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
 	if (should_fail_alloc_page(gfp_mask, order))
 		return NULL;
 
-	/*
-	 * Check the zones suitable for the gfp_mask contain at least one
-	 * valid zone. It's possible to have an empty zonelist as a result
-	 * of __GFP_THISNODE and a memoryless node
-	 */
-	if (unlikely(!zonelist->_zonerefs->zone))
-		return NULL;
-
 	if (IS_ENABLED(CONFIG_CMA) && ac.migratetype == MIGRATE_MOVABLE)
 		alloc_flags |= ALLOC_CMA;
 
@@ -3394,8 +3386,6 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
 	/* The preferred zone is used for statistics later */
 	preferred_zoneref = first_zones_zonelist(ac.zonelist, ac.high_zoneidx,
 				ac.nodemask, &ac.preferred_zone);
-	if (!ac.preferred_zone)
-		goto out;
 	ac.classzone_idx = zonelist_zone_idx(preferred_zoneref);
 
 	/* First allocation attempt */
@@ -3418,7 +3408,6 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
 
 	trace_mm_page_alloc(page, order, alloc_mask, ac.migratetype);
 
-out:
 	/*
 	 * When updating a task's mems_allowed, it is possible to race with
 	 * parallel threads in such a way that an allocation can fail while
-- 
2.6.4